The following discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was published, known or part of the common general knowledge in any jurisdiction as at the priority date of the application.
Potentiation of gene transcription by nuclear hormone receptors involves its interplay with co-activators and the basal transcriptional machinery through protein-protein interactions (1, 2). Co-activator can act as transcriptional adaptor or modify chromatin through histone acetyl transferases (HAT) or nucleosome remodeling complexes. Furthermore, coactivators regulate mRNA transport, translation and posttranslational modification of the synthesized proteins. Some of the known nuclear hormone receptor coactivators include p160 family members of co-activators, SRC-1, SRC-2[TIF-2/GRIP-1/NCoA-2], SRC-3 [pCIP/ACTR/AIB-1/RAC-3/TRAM-1], NRIF-3, E6-AP and WBP2. Due to their pleiotropic roles, it is not surprising that transcription co-activators are emerging as a group of proteins increasingly implicated in cancer development (3-5).
Transcription co-activators are often subject to posttranslational modification, e.g. phosphorylation. Phosphorylation of specific members of SRC/p160 family of proteins enhanced their nuclear localization (6), inhibited their interactions with non-nuclear receptor activators (7) or stimulated their intrinsic coactivator activity (8). Phosphorylation of AIB1 and PGC-1 regulated both their half-life and activity (9, 10). Furthermore, phosphorylation of NRIF3 via Pak1 promoted ERα transactivation through increased ERα-NRIF3 interaction (5).
WW-domain binding protein (WBP2) is a transcription coactivator demonstrated to selectively and specifically enhance ERα and PR transactivation via hormone-dependent ERα/PR-WBP2 interaction and recruitment of WBP2 to hormone-responsive elements (11). WBP2 contains an intrinsic activation domain. One of its three polyproline (PPXY) motifs-PY3 is essential for its coactivating function in ERα/PR transactivation. Its coactivator activity could be further enhanced by YAP (Yes kinase-associated protein), which also regulated several transcription factors, e.g. p73 (12), Runx2 (13), TEAD/TEF (14) and ErbB4 (15).
Our previous study has identified WBP2 as a novel tyrosine kinase substrate that displayed differential phosphorylation across the MCF10AT model of breast cancer progression (16). Exogenously expressed WBP2 was subsequently validated to be an authentic target of EGFR. We hypothesized that EGFRmediated tyrosine phosphorylation of WBP2 plays a role in regulating ERα function and breast cancer biology. We therefore attempted to delineate the signaling pathways for the EGFR-mediated tyrosine phosphorylation of WBP2 and to study the impact of WBP2 phosphorylation on its coactivator activity. The role of WBP2 and its tyrosine phosphorylation on the ER-positive breast cancer biology and the underlying mechanisms were also investigated.